With an internal combustion engine with direct fuel injection the fuel is injected by means of an injection valve directly into the combustion chamber. To start such an internal combustion engine both a low-pressure start and also a high-pressure start can be carried out. With a low-pressure start the fuel is injected at a typical pressure of 5 to 8 bar. At low temperatures in particular this process results in parts of the fuel being deposited onto the cold combustion chamber walls, the so-called wall film. These fuel deposits are involved only partly or not at all in the combustion process. The result is an increase in fuel consumption and emission of pollutants during the start process. For this reason the start process of the internal combustion engine is preferably carried out as a high-pressure start, in which the fuel is injected at a considerably higher pressure, typically 20 to 30 bar, into the combustion chamber. The finer atomization of the fuel significantly reduces the deposition of fuel at lower temperatures, which has a positive effect on the fuel consumption and pollutant emissions. The problem with a high-pressure start however is a sufficiently fast and stable pressure buildup in the injection system, to ensure a rapid start and a stable combustion.
Application DE 10 2004 029 378 A1 has disclosed a method for starting an internal combustion engine in which even before the starter is actuated pressure is built up in the injection system by operation of the fuel pump. It is further proposed in this publication that on actuation of the starter the crankshaft is initially started into rotation without an injection being triggered. The result achieved is that a sufficiently high pressure is built up by means of the fuel pump coupled with the crankshaft before first injection is started.
It is further known that the injection is enabled in the injection system on reaching a pressure threshold value, which indicates that sufficiently high pressure obtains.
Although the known method achieves an early buildup of pressure in the injection system and thereby a rapid enabling of the injection, it can however result after the starting of the first injection in a pressure drop in the injection system and thus to an uncomfortable start behavior of the internal combustion engine.